Remember all those polls that predicted Barack Obama would defeat Hillary Rodham Clinton in the New Hampshire Democratic presidential primary 15 months ago? A committee of polling experts on Monday issued a detailed report laying out some reasons why all the surveys were off.

One explanation is that the surveys of the likely New Hampshire Democratic electorate ended too early to detect a primary-eve shift in opinion toward Clinton, who won the contest by 3 percentage points. “There is compelling evidence that this was an important contributor to the error,” said the report, which was compiled by the American Association for Public Opinion Research.

The report also said that Clinton supporters may have been underrepresented in voter samples. Clinton did well among voters with lower levels of formal education and income, who may have been tougher to reach by telephone than more upscale Obama voters.